Feb. 15, 2011 -- Do you drink a nightcap to help you sleep? It may not be as effective as you think, new research suggests.

One of the largest studies to date on alcohol's effects on sleep shows that drinking alcohol before bed may disrupt sleep and increase wakefulness in healthy adults -- affecting women more than men -- regardless of family history of alcoholism.

The research is reported in the May 2011 issue of Alcoholism: Clinical & Experimental Research.

In the women studied, alcohol decreased sleep duration and efficiency (ratio of time sleeping in bed to total time spent in bed) and increased how often they woke up during the night. Alcohol deepened sleep during the first half of the night but then disrupted sleep during the second half of the night, a finding that previous studies have reported.

Study researcher J. Todd Arnedt, assistant professor of psychiatry and neurology at the University of Michigan, says in a news release: "It's clear that a substantial portion of the population uses alcohol on a regular basis to help with sleep problems. This perception may relate to the fact that alcohol helps people fall asleep quickly and they may be less aware of the disruptive effects of alcohol on sleep later in the night."

The study included 93 healthy adults in their 20s (59 women and 34 men) who were college students or recent college graduates; 29 of the participants had a family history of alcoholism.

The study took place over two nights. On the first night, researchers randomly gave participants an alcoholic beverage containing either vodka or bourbon mixed with caffeine-free cola. On the second night participants were given a "placebo" beverage with a few drops of bourbon or vodka floated on top.

The researchers instructed them to drink to intoxication. After researchers measured their breath alcohol concentration (BrAC), participants slept for eight hours between 11 p.m. and 7 a.m.

The participants were monitored during their sleep. Participants completed questionnaires on sleepiness and sleep quality before bedtime and when awakened the next morning.

Alcohol and Sleep: Men vs. Women

The findings showed that alcohol disrupted sleep more in women than in men at equivalent BrACs. Women's total sleep time was reduced by 19 minutes, sleep efficiency decreased by 4%, and there was a 15-minute increase in the time they spent awake during the night after drinking alcohol, compared to the placebo.

"It is important to note that the peak BrACs were equivalent between men and women in our study, so the findings are not due to higher BrACs among the female subjects," Arnedt says in the news release.

Sleep continuity following alcohol compared to placebo was not significantly different in men. Researchers did not find differences in sleep measures among participants with a family history of alcoholism.

"These differences may be related to differences in alcohol metabolism," according to Arnedt, "since women show a more rapid decline in BrAC following alcohol consumption than men."

"We also do not believe that the differences," Arnedt says, "were due to differences in alcohol experience because the prior alcohol use was also equivalent between the men and women."

Further study may help researchers understand more about the relationship between sleep quality and the development of alcohol use disorders, he says.